Epitaxial CaGe₂ films on germanium

The epitaxial growth of thin CaGe₂ films with reactive deposition epitaxy (RDE) on Ge(1 1 1) substrates is described. The films consist in general of a mixture of the known trigonal rhombohedral tr6 modification and a hexagonal h2 modification of CaGe₂ containing two Ca and two buckled Ge layers per unit cell in a twofold stacking sequence whose formation appears to be favored by strain. Epitaxial layers of both polytypes show remarkably higher crystalline quality compared to epitaxial CaSi₂ films grown on silicon substrates. The tr6 modification is found to be unstable in air in contrast to the h2 modification.